Photo-lithography is the conventional lithographic method used for manufacturing miniaturized electronic circuits. In this method, a photoresist layer having a predetermined thickness is deposited upon a surface, usually a semiconductor wafer surface. The deposited resist layer then is patternwise exposed, typically by irradiating it with light through a mask which carries the pattern of the circuitry to be formed. The exposed resist layer is then developed. Photo-lithography has a resolution (the smallest size which can be defined) limited by diffraction effects, which are in turn related directly to the wavelength of the light used for making the exposure.
Higher resolution is available by using electron beam lithography. In electron beam lithography, a deposited electron-sensitive resist layer is patternwise exposed, typically using a scanning electron beam that is turned on and off so as to form an exposure image in the resist which corresponds to the circuitry desired. The electron beam exposed resist is then developed. In scanning electron beam lithography, resolution is limited chiefly either by electron scattering effects in the material being irradiated (if the beam diameter is small enough) or by the diameter of the electron beam (if the beam diameter is too large).
The need for high-performance integrated circuits, memories and similar devices requires ever finer resolution in the lithographic processes. Although it has been realized in theory that electron scattering effects may be reduced by lowering the energy of the electrons in an electron beam, the minimum achievable beam diameter in conventional electron beam machines increases as the energy of the electrons in the beam is reduced because of chromatic aberration in the magnetic and/or electronic lenses, among other causes. Consequently, as the energy of the electrons in a conventional electron beam is reduced, the resolution actually deteriorates rather than improves as a result of the increasing beam diameter even though electron scattering itself becomes less of a problem.